border-radius: x / y 分别定义了半长轴和半短轴。两个一起组成了一个椭圆
<html>
<head>
<style>
    .cur{
        width: 500px; 
        height: 100px;  
        border: solid 5px #000;
        border-color: #000 transparent transparent transparent;
        border-radius: 50%/100px 100px 0 0;
    }
</style>
</head>
<body>
    <div class="cur"></div>
</body>
</html>